Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle mounted on the tower, and a rotor coupled to the nacelle. The rotor typically includes a rotatable hub and a plurality of rotor blades coupled to and extending outwardly from the hub. The rotor blades capture kinetic energy of wind using known airfoil principles. More specifically, the rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to the gearbox, or if the gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a power grid.
In a wind turbine generator, such as a doubly fed induction generator (DFIG), a stator is directly connected to the power grid, and a rotor is connected to the power grid via an AC-DC-AC power converter. When the generator is in the power generating mode, an electromagnetic (EM) torque of the generator is controlled by a controller to match a mechanical torque of the wind turbine. If a sudden grid loss event or failure of the converter occurs, the converter loses the ability to control the EM torque, and the EM torque is reduced to zero within 100-200 milliseconds. In contrast, it takes tens of seconds to a couple of minutes for the mechanical torque to reduce to zero when the mechanical torque is reduced only by mechanical operation of pitching out blades of the wind turbine. Due to the sudden loss of EM torque and the slow decaying of the mechanical torque, the rotor may be accelerated to exceed a rated speed even when the blades of the wind turbine are pitched out at the fastest rate feasible. The acceleration of the rotor combined with the loss of aerodynamic thrust due to fast pitching of blades results in high loading on the turbine mechanical structure, especially on the tower, blades, and hub. Therefore, the need to withstand the sudden EM torque loss event usually drives the design of most of wind turbine components.
Large rotors continue to be the most dominant trend in wind industry in recent years as they drive attractive project economics. But, larger rotors, with the heavier mass and higher inertia, lead to increased loads on the turbine mechanical and structural components. It is observed that the maximum loading on the turbine mechanical components is determined by how well the rotor over speeding is controlled during a shut-down event in response to extreme fault of sudden loss of counter torque. As such, for some systems, a mechanical brake is placed on the high-speed shaft to reduce the rotor over speeding. However, the mechanical braking system has certain drawbacks such as sub-rated brake torque (˜0.5 pu), slower kick-in time (3-4 seconds), as well as wear and tear of its components.
Thus, enhanced braking capability combined with the existing mechanical brake system, could help the wind turbine better manage the loads during the extreme events. In addition, in an effort to provide a smoother turbine shutdown, some modern systems employ a 10-second torque buffer; however, such systems assume converter availability. Thus, there is a need for an improved braking system that addresses the aforementioned issues.